If a flag is declined without any explanation, that always means it was declined from the review queue. A flag put a post in the close-vote or low-quality review queue, and all(1) reviewers decided the other way. In this case, all three reviewers voted to leave the question open, and thus your flag was automatically declined. Nobody ever had the chance to explain why the flag was declined - nobody was aware that it would be declined until after the fact.
When a moderator declines a flag, there is always a message (usually one of the canned messages, but often we give individual decline reasons) why the flag was declined; the software doesn't allow a moderator to decline a flag without giving a reason.
(1) I think one needs a unanimous outcome of the review to decline a flag, if the outcome is mixed, the flag is disputed. But I'm not 100% sure about that, could be that a large enough majority suffices to decline.